1. Field of the Invention
This invention relates to filamentary multilayer superconductor articles. The invention also relates to superconductor articles suitable for use in alternating current (ac).
2. Background of the Invention
Since the discovery of high-temperature superconducting (HTS) materials (superconducting above the liquid nitrogen temperature of 77 K) there have been efforts to develop various engineering applications using such HTS materials. In thin film superconductor devices and wires, significant progress has been made with fabrication of devices utilizing an oxide superconductor including yttrium, barium, copper and oxygen in the well-known basic composition of YBa2Cu3O7-x (hereinafter referred to as “YBCO”). Biaxially textured superconducting metal oxides, such as YBCO, have achieved high critical current densities in a coated conductor architecture, often referred to as second generation HTS wires, or a “coated conductor.” The expression “HTS wire” indicates a HTS conductor with the attributes that make it useful for the construction of a superconducting device; its cross-sectional geometry can vary from tape-like to round.
Typically, second generation HTS wires 10 include a metal substrate 11, buffer layer(s) 12, and an active layer 13, e.g., a superconductor, as illustrated in FIG. 1. The metal substrate, such as Ni, Ag, or Ni alloys, provides flexibility for the article and can be fabricated over long lengths and large areas. The buffer layer(s) consists of metal oxide layers, such as LaAlO3, Y2O3, CeO2, or yttria-stabilized zirconia (YSZ); it makes up the next layer and serves as a chemical barrier layer between the metal substrate and the active layer. The buffer layer(s) reduces oxidation of the substrate and also reduces the diffusion of chemical species between the substrate and the superconductor layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion that is well matched with the superconductor material.
To achieve high critical current densities in the wire, the superconducting material has a sharp biaxial texture. As used herein, “biaxially textured” refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface and a direction perpendicular to the surface. One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Cube textured metal foils such as Ni or Ni alloys can serve as a substrate and texture template for high quality HTS wires.
When using a cube textured substrate the buffer layer is an epitaxial layer, that is, its crystallographic orientation is directly related to the crystallographic orientation of the substrate surface onto which the buffer layer is deposited. For example, in a multi-layer superconductor having a substrate, an epitaxial buffer layer and an epitaxial layer of superconductor material, the crystallographic orientation of the surface of the buffer layer is directly related to the crystallographic orientation of the surface of the substrate, and the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the surface of the buffer layer.
Second generation HTS wire can be incorporated into a variety of devices for many applications, including cables, motors, generators, synchronous condensers, transformers, current limiters, and magnet systems. The incorporation of second generation superconducting YBCO wires into such devices provides the opportunity to dramatically reduce the device cooling requirements, thus enabling the development of lightweight, compact, high-power sources. Currently a wide, e.g., several millimeters, tape configuration is used to reach practical electrical currents.
Many potential applications for HTS wires involve operating the superconductor in the presence of ramped magnetic or oscillating magnetic fields, or require that the HTS wire carry alternating current. In the presence of time-varying magnetic fields or currents, there are a variety of mechanisms that give rise to energy dissipation, hereinafter referred to as “ac losses.” Although second generation HTS wire is currently suitable for many types of electric power devices, the ac losses from the current HTS wires are too high for use in demanding HTS applications in which the alternating magnetic fields have a higher amplitude or frequency. The use of an HTS wire with greatly reduced ac losses would enhance the application of these wires in a great variety of novel, HTS-based devices.
There are a number of factors contributing to the total ac loss in a superconducting wire, such as superconducting properties and dimensions of the superconducting oxide film, and the electrical and magnetic properties of the metal constituents of the conductor. A major contributor to the ac losses is so-called hysteretic losses in the superconducting oxide film caused by an oscillating external magnetic field. This loss contribution is proportional to the film width as seen by the magnetic field direction, and is therefore greatest when the magnetic field is perpendicular to the film surface, or when the alternating magnetic field has a large perpendicular component. For current HTS superconductor widths even a moderate ac frequency and magnetic field perpendicular to the superconducting film plane can produce very large ac losses. It has been proposed to divide an oxide superconducting film into narrow filaments to suppress ac loss in a superconducting oxide thin film.
FIG. 2 is a perspective view of a portion of a coated conductor article in which the superconducting film is arranged as a thin filament array. The multilayer article 20 includes a metal substrate 21 having a textured surface and epitaxially grown buffer layer(s) 22. Such textured bases have been previously described. A RABiTS™ (rolling-assisted, biaxially textured substrates) textured template is typically used. A RABiTS™ substrate is a roll-textured and annealed metal tape, e.g., nickel or nickel alloy such as NiW with a sharp cube texture, covered in an epitaxial manner with one or more oxide or metal buffer or conditioning layers. Another variation used to prepare the textured template is ion beam assisted deposition or IBAD. The resulting textured base serves as a template for the HTS compound, e.g., yttrium-barium-copper-oxide (YBCO). Superconductor filaments 23 run substantially continuously along the length of the base to form an array of substantially parallel filaments. The superconducting filaments are crystallographically oriented and typically exhibit biaxial texture.
Short sample testing of a superconductor article patterned into multiple filaments shows a reduction in ac loss proportional to the reduction in conductor or filament width when exposed to an alternating magnetic field with a perpendicular field component. In principle the filaments can be electrically isolated from each other and the absence of a conductive path would strongly reduce so-called interfilamentary coupling losses.
Second generation HTS wire production is based on a variety of continuous reel-to-reel thin film deposition techniques, practiced over very long lengths as superconducting wires are needed in piece lengths that can reach 1000 meters. Small defects can locally disrupt current transfer, and their effect becomes more serious when the conductor width is reduced. In narrow filaments of, for example, 100 micrometer width, small defects can potentially seriously disrupt local current transfer and render the conductor useless when used in long lengths. A certain degree of current sharing capability between filaments is therefore desired to mitigate the effect of these small defects, allowing currents an alternative path in case of an occasional local current constriction.